K. Tedd et al. (2008-FS-WQ-16-S4)
5
Hydrogeology
5.1
Introduction
•
Dinantian Dolomitised Limestones (Waulsortian,
Butlersgrove
The
solid
geology
underlying
the
SERBD
is
and
Milford
Formations
and
equivalent horizons);
heterogeneous. In general, alteration, cementation
•
and intensive structural deformation mean that
Dinantian Karstified Limestones (Ballyadams and
Clogrennan Formations); and
groundwater flow will be through secondary porosity
and will be dominated by fracture flow. In a typical
•
fractured hydrogeological system, the occurrence of
open water-bearing fractures is greatest at shallow
Quaternary Sand and Gravel aquifers.
The locally important aquifers in the study area are the:
depths. Typically, the hydraulic conductivity declines
•
with depth as fractures become tighter and less
Devonian
to
Early-Carboniferous
Kiltorcan
Sandstone Aquifer – in upland areas;
common. Therefore, the groundwater flow paths are
likely to be shallow, predominantly in the upper layer of
•
the aquifer with enhanced weathering and open
Waulsortian Limestone, where it is not dolomitised
or highly fractured;
fractures (Robins and Misstear, 2000). However, the
karstified and dolomitised limestones may have
•
Dinantian Crosspatrick Formation;
•
Dinantian Argillaceous Limestones (Ballysteen
permeable zones at greater depths, relating to earlier
periods of alteration.
Formation);
Extensive subsoil deposits overlie the solid geology. In
•
contrast to bedrock aquifers, sand and gravel aquifers
Westphalian Sandstones (Lickfinn Coal Formation
and equivalent horizons); and
provide an opportunity for intergranular groundwater
flow, which may, in some cases, be of regional
•
Quaternary Sand and Gravel aquifers.
importance. Tills are the most widespread subsoil
The poor aquifers in the study area are the:
deposit and, while they do not form aquifer units, they
may influence the hydraulic conditions of, and the
•
proportion of recharge reaching, any underlying
Lower Palaeozoic strata, excluding Ordovician
Volcanics;
aquifers.
5.2
Aquifer Properties
Figure 5.1 shows the distribution of aquifer types within
•
Leinster Granites; and
•
Namurian Sandstones and Shales.
5.2.1
the study area, Table 5.1 outlines the properties of the
aquifer units within the study area and Box 5.1 outlines
Regionally important aquifers
5.2.1.1 Ordovician Volcanics
The Ordovician Volcanics are classified as a regionally
important fissured aquifer (Rf).
the GSI’s aquifer classification system.
The regionally important aquifers in the study area are
The dominant hydraulic conductivity, or permeability,
(GSI, 2006):
in these strata is secondary, along joints developed
•
Ordovician volcanics;
•
Devonian
during the Caledonian and Variscan orogenies. Well
logging in south-east County Kilkenny indicates that
Kiltorcan
developed fissures occur down to 50 m. It is likely that
Sandstone Aquifer (including sandstones of the
there will be very little natural flow in these aquifers at
overlying Porter’s Gate Formation);
depth (Daly, 1982).
to
Early-Carboniferous
23
Review of groundwater-level data in the SERBD
Figure 5.1. Aquifer types of the South Eastern River Basin District.
24
Table 5.1. Properties of the aquifer units within the study area. (Fm, formation; for aquifer category codes, please refer to Box 5.1.)
Geological
age
Group
Quaternary
Aquifer name
Hydrogeologically
significant formations
Sand and
gravel1
Aquifer
category
Thickness
(m)
Lg
Well yield3
(m3/day)
Specific capacity3
(m3/day/m)
Transmissivity3
(m2/day)
Typical
Range4
Typical
Range
Typical
Range
500
200–1,000
200
50–1,000
400
100–2,000
Hydraulic
conductivity5
(m/day)
Specific
yield6
(–)
1–250
0.05–0.15
Namurian
Westphalian
vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
Deltaic cycles of
coals and thick
sandstones
Westphalian
Sandstones1
Sandstones, shales
and limestones
Namurian
Sandstones and Bregaun Flagstone Fm
Shales1,7
Lickfinn Coal Fm/
Clay Gall Sandstone Fm
Lm
<350/250–320
200–500
100–1,100
10–50
5–50
390–460
20–50
10–200
1–5
0–15
5–15
1–500
0.001–50
0.01
Pl
Killeshin Siltstone Fm
Pl
Luggacurran Shale Fm
Pu
0.005–0.01
Carboniferous
Karstified
Limestones1,7
Visean Limestones
25
Dinantian
Crosspatrick
Fm1
Waulsortian
Mudbank Complex
Dolomitised
Limestones1,7
Sub-Waulsortian
Limestones
Clogrennan Fm
Rkd/Lk
Ballyadams Fm
Rkd/Lk
Devonian
Tullow and
Blackstairs Granites
20–500
10–2,000
5–100
1–3,000
200
5–3,000
0.1–100
0.005–0.05
<60
250–400
100–500
10–20
10–50
20–40
10–100
0.1–10
0.01–0.02
50–220
1,000
300–3,000
100
10–350
250
20–800
0.1–20
0.02–0.04
300–500
(20–40)
(10–150)
(2–10)
(0–100)
500
50–1,300
40
2–270
Ll
Aghmacart Fm
Ll
125–160
Lm
Waulsortian Limestones
Rkd, Lk
(Ll)
Ballysteen Fm
Rkd, Rf
(Ll)
Ballymartin Fm
Rf, Lm
(Ll)
Crosspatrick Fm
Ballyvergin Fm
Lower Carboniferous
Sandstones and
Shales
Kiltorcan
Sandstone1
Old Red
Sandstone
225–400
Durrow Fm
3–5
Porter's Gate Fm
Rf
30–80
Kiltorcan Fm
Rf
20–230
Granites
(0.01–0.03)
60
10–500
0.1–10
0.01–0.02
Pl/Ll
vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
Lower
Palaeozoic
1
Metasedimentary
and volcanics strata
Ordovician
Volcanics2
Campile Fm
Rf
400–1,500
200–2,000
After Daly (1994).
Daly (1982).
3Derived from pumping tests on wells that are mainly partially penetrating.
4
Tests are not practical where the yield is less than 10 m3/day.
5
An average value over the productive section of pumped wells.
6
Obtained from pumping tests, core measurement or hydrograph analysis.
7
Boreholes failing to provide a minimum domestic supply (10 m3/day) are not uncommon in these formations.
2After
5–200
15 to >500
0.01
K. Tedd et al. (2008-FS-WQ-16-S4)
vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
Review of groundwater-level data in the SERBD
Box 5.1. Aquifer Classification
Based on the hydrogeological characteristics and on the value of the groundwater resource, all of the Republic of
Ireland’s land surface is divided into nine aquifer categories. Eight of the aquifer categories are defined in
Groundwater Protection Schemes (DELG/EPA/GSI, 1999), and they are as follows:
Regionally Important (R) Aquifers
1. Karstified bedrock (Rk)
2. Fissured bedrock (Rf)
3. Extensive sand & gravel (Rg)
Locally Important (L) Aquifers
4. Bedrock which is Generally Moderately Productive (Lm)
5. Bedrock which is Moderately Productive only in Local Zones (Ll)
6. Sand & gravel (Lg)
Poor (P) Aquifers
7. Bedrock which is Generally Unproductive except for Local Zones (Pl)
8. Bedrock which is Generally Unproductive (Pu)
During the course of the National Aquifer Classification Programme (completed in 2004) undertaken for the Water
Framework Directive, a further aquifer category was established:
9. Locally important karstified bedrock (Lk)
Note that, depending on the degree and nature of the karstification, regionally important karstified bedrock aquifers
(Rk) may be further characterised as either:
•
Regionally important karstified bedrock aquifers dominated by conduit flow (Rkc); or
•
Regionally important karstified bedrock aquifers dominated by diffuse flow (Rkd).
The volcanic units close to outcrop are unconfined but
Sandstone Aquifer, fracturing tends to be restricted to
the aquifer becomes confined as it dips under the
the
overlying interbedded sediments (Daly, 1982).
permeability and porosity, are more extensive in the
sandstone
units.
Fractures,
and
hence
south and are more intense around major structural
5.2.1.2 Kiltorcan Sandstone Aquifer
features such as faults and anticlines (Daly, 1988).
The Kiltorcan Sandstone Aquifer is classified as a
regionally important fissured bedrock aquifer (Rf). Due
Fracturing is likely to be considerably reduced at depth
to a higher proportion of sandstone, it is expected that
and may be absent in the centre of large synclines
the permeability of the aquifer will be greatest in the
where the sandstones are in compression. A borehole
upper half of the Kiltorcan Formation and at the base
log for Knocktopher Manor (Kny 31/73, i.e. the
of the Porter’s Gate Formation (Daly, 1988).
pumping borehole to which Kny 31/72 is an
The secondary porosity and permeability of the
observation borehole) (Daly, 1994) shows major
Kiltorcan Sandstone Aquifer is produced by fracturing
inflows between 35 m and 39 m and between 42 m and
and, to a lesser extent, weathering. In the Kiltorcan
52 m. Evidence from other boreholes has established
26
K. Tedd et al. (2008-FS-WQ-16-S4)
that productive fracturing extends to depths of over
The extent of the dolomitisation within the SERBD is
100 m.
quite variable. Dolomitisation appears to decrease
from east to west. In the east (Callan to Bennettsbridge
Many of the minerals of the Kiltorcan Sandstone
Lowlands), the boundary of the aquifer, i.e. the extent
Aquifer are susceptible to chemical weathering.
of the dolomitisation, can extend from the Aghmacart
Weathering is pronounced along fractures and results
Formation, through the Crosspatrick Formation and
in the sandstone being friable. The total porosity of the
Waulsortian Limestones to the Sub-Waulsortian
sandstone is normally less than 5%; however, in
Limestones. In the west (Urlingford to Mountrath
heavily weathered sections the porosity may be in
Lowlands), however, the aquifer is restricted to the
excess of 10% (Daly, 1988).
Waulsortian Limestones (Daly, 1993).
Pumping tests, conducted in 1980, on two boreholes at
The interpretation of geophysical logs for the Rathduff
Knocktopher Manor (Kny 31/72 and Kny 31/73) gave
Borehole (Daly, 1994) shows 18 m of subsoils
m2/day
and 1,789
overlying 7 m of limestone, with occasional dolomitised
m /day, respectively, and storage coefficient estimates
horizons, overlying 33 m of dolomitised limestones,
transmissivity estimates of 1,910
2
of 3.9 ×
10–6
and 7.2 ×
10–4,
with occasional undolomitised limestones.
respectively, for the
confined conditions (Daly, 1994).
Daly (1994) suggests that this aquifer may have
significant amounts of deep (>200 m) groundwater
The rock formations above and below this aquifer
flow, based on the following:
confine it over much of its extent. In the more low-lying
outcrop areas, where the overlying subsoils are often
•
The aquifer is continuous at depth;
•
There is significant permeability at depth;
•
Head
thicker, the aquifer may be confined by till or in
continuity with sands and gravels. Daly (1987)
describes artesian conditions in this aquifer to the east
of Callan. Water-table maps for this aquifer indicate
that
the
main
discharge
areas
are
in
difference
Mountrath
the
between
Lowlands
the
and
Urlingford
the
Callan
to
to
Bennettsbridge Lowlands is in excess of 30 m and
Knocktopher/Ballyhale area and at Thomastown in
could provide the hydraulic drive;
South Kilkenny, and at Rushin Crossroads in the
Slieve Bloom area (Daly, 1994).
•
Large springs discharging in the Callan to
Bennettsbridge Lowlands; and
5.2.1.3 Dinantian Dolomitised Limestone Aquifer
•
The Dinantian Dolomitised Limestone Aquifer is
of water from this aquifer in the Callan to
classified as either Rkd or Lk, depending upon the
Bennettsbridge Lowlands.
extent of dolomitisation. The aquifer comprises the
Sub-Waulsortian
Limestones,
Waulsortian
5.2.1.4 Dinantian Karstified Limestone Aquifer
The Dinantian Karstified Limestone Aquifer is
classified as Rkd or Lk, depending upon the extent of
karstification. The secondary permeability and porosity
is developed by karstification of the Visean
Limestones.
Limestones, the Crosspatrick Formation and the
Aghmacart Formation where dolomitised (Daly, 1994).
Where not dolomitised, the formations are classified as
Ll and are dealt with in Section 5.2.2. Of these units,
only the Crosspatrick Formation has significant and
consistent
permeability
in
the
absence
Slightly elevated temperatures of some discharges
of
The Dinantian Karstified Limestone Aquifer is bounded
dolomitisation (Daly, 1993).
by
the
underlying
locally
important
Aghmacart
Dolomitisation is the replacement of calcite with
Formation and the overlying generally unproductive
dolomite. It is important hydrogeologically because it
Luggacurren
results in an increase in porosity (and permeability), as
karstified sections of the intervening strata are
the crystal lattice of dolomite occupies about 13% less
classified as part of the aquifer. Daly (1994) describes
space than that of calcite (Freeze and Cherry, 1979).
the limit of karstification development as the lowest
27
Shale
Formation.
However,
only
Review of groundwater-level data in the SERBD
level to which the strata could have drained in
This aquifer discharges via numerous springs close to
geological history. He suggests this level to be around
the main river channels or directly into the rivers
30 maOD in the Urlingford to Mountrath Lowlands and
themselves. The aquifer discharges in the Nore just
close to 0 maOD in the Callan to Bennettsbridge
north of Bennettsbridge (Daly, 1994), in the Nuenna
Lowlands. The aquifer is not believed to have any
via a number of springs including the Boiling Well at
significant permeability where it lies beneath the
Clomantagh, in the Suir Catchment at the Kiltinin
Namurian strata (Daly, 1994).
Castle Springs, which flow into the Anner River, and in
the Barrow Catchment via sands and gravels between
Cawley
(1990)
investigated
the
hydrology
Carlow and Leighlinbridge (Daly, 1982).
and
hydrogeology of the Nuenna/Borrismore Catchment, a
minor tributary of the Nore River. Three aquifer zones
5.2.2
Locally important aquifers
were identified and the aquifer properties for each
5.2.2.1 Dinantian Crosspatrick Formation
zone are presented in Table 5.2. Cawley also modelled
The Crosspatrick Formation is classified as an Lm
a significant proportion of groundwater discharge,
aquifer. This long narrow limestone aquifer occurs in
particularly in winter, as ‘quickflow’; this supports other
the Urlingford to Mountrath Lowlands. It is not well
evidence of conduit flow in this aquifer (Daly, 1994)
defined in the Callan to Bennettsbridge Lowlands
owing to extensive dolomitisation and, therefore, is
There are numerous karst features within this aquifer.
included as part of the Dinantian Dolomitised
In the Nuenna Catchment, which includes the
Limestone Aquifer. Where not dolomitised, this
Woodsgift, Clomantagh Lower and Tubbrid Lower
formation forms part of the Dinantian Pure Bedded
cluster of MPs (see Fig. 1.1 for location), there are
Limestone rock unit.
many mapped karst features within the GSI karst
features database. For example, within a 10-km radius
In the south-west and centre of the Urlingford to
of these MPs there are: one cave, two enclosed
Mountrath Lowlands, the aquifer is bounded at the
depressions, 28 springs (located where the subsoil is
base by the Dinantian Dolomitised Limestone Aquifer
thin or permeable), seven swallow holes (located
with which it is in hydraulic continuity. In the north-east,
especially along the Namurian–Dinantian contact
it is bounded at the base by the Sub-Waulsortian
south-east of Johnstown) and one turlough. In
Limestones. The Aghmacart Formation, an Ll aquifer,
contrast, where the aquifer is exposed to the north of
forms the upper boundary (Daly, 1994).
the Castlecomer Plateau (where the Land Commission
Over the outcrop area, the hydraulic conditions vary
and Masterson Boreholes are located), there are fewer
from unconfined, with hydraulic continuity to the
mapped karst features, e.g. three springs and three
overlying sands and gravels, to being confined by thick
caves within a 10-km radius of the boreholes.
till. It becomes confined down dip where it is likely to be
much less permeable unless dolomitised (Daly, 1994).
An unlabelled geophysical log for the Woodsgift
Borehole (Kny 12/8) (unpublished GSI records)
At the Granston Manor Borehole the formation is
indicates inflows from fissures or fractures at depths of
partially dolomitised. The geological log (Daly, 1994)
19.2 mbgl, 25.4 mbgl and 30.0 mbgl.
notes water inflows at 6 mbgl coincident with sand
Table 5.2. Aquifer properties within the Nuenna/Borrismore Catchment (after Cawley,
1990).
Zone
Transmissivity
(m2/day)
Specific yield
(%)
Well yield
(m3/day)
1–5
0.5–1.0
<50
Intermediate zone
120–150
1.0–2.5
<750
Discharge zone
150–750
3.0–5.0
<2,000
Recharge zone
28
K. Tedd et al. (2008-FS-WQ-16-S4)
subsoils, 10 mbgl coincident with loose rock at the top
5.3
Subsoils
5.3.1
Quaternary Sand and Gravel aquifers
of the bedrock and a major inflow at 24 mbgl coincident
with a large fracture.
There are a number of regionally and locally important
The aquifer is recharged in more elevated areas,
sand and gravel aquifers within the study area (see
where Quaternary deposits are thin and permeable,
Fig. 5.1).
and discharges into small streams (Daly, 1994).
The Mid-Kildare and Kilmanagh Sand and Gravel
5.2.2.2 Westphalian Sandstones
Aquifers are classified as regionally important aquifers
The Westphalian Sandstones are classified as an Lm
(Rg). A number of other sand and gravel deposits in
aquifer. Due to the overlying clays, shales and
the study area associated with the main channels of
siltstones, the sandstone aquifer units are confined
the Nore, Barrow and Suir are also substantial enough
apart from at, or near, an outcrop.
(>10 km2 in area and >5 m saturated thickness) to be
classified as Rg aquifers. Smaller deposits within the
Pumping tests conducted on two boreholes in the
study area are classed as locally important sand and
Slieveardagh Hills (including the Ballincurry Borehole
gravels aquifers (typically 1–10 km2 in area).
Tip 55/65) gave transmissivity values of between
10 m2/day and 101 m2/day. There were artesian
In contrast to the bedrock aquifers, sand and gravel
conditions in the Ballincurry Borehole (Tip 55/65); test
aquifers
results gave an average storage coefficient of 4.4 ×
primary
porosity
and
flow
is
intergranular. The hydraulic conditions in these
10–4. Safe yields for the two boreholes were calculated
deposits are quite variable, depending upon lithology
to be 360 m3/day and 900 m3/day, respectively (Daly,
and stratigraphy. Confined and unconfined conditions
1980). The hydraulic conditions of the Westphalian
are often found within a short distance and the aquifers
Sandstones within the Slieveardagh Hills and the
are frequently in continuity with rivers and the
Castlecomer Plateau are mostly confined (Daly, 1980;
underlying aquifers. As such, the sands and gravels
Daly et al., 1980).
can represent an important source of additional
storage for the underlying aquifers (Daly, 1982). Due to
5.2.2.3 Lower Carboniferous
the limited extent of some of these deposits, flow paths
The Sub-Waulsortian and Waulsortian Limestones,
are often short and groundwaters discharge into
where not dolomitised, the Durrow Formation, where
streams with which they are in contact (Daly, 1994).
not karstified, and the Aghmacart Formation are
classified as locally important aquifers. The areas
5.3.2
underlain by these strata are frequently poorly drained,
especially in the Urlingford to Mountrath Lowlands
Subsoil permeability
vulnerability
and
groundwater
Even subsoils which do not form aquifer units may be
(Daly, 1994).
of
5.2.3
have
Poor bedrock aquifers
hydrogeological
depending
upon
importance.
thickness,
For
and
example,
particularly
The early Palaeozoic strata and the Leinster Granites
permeability, subsoils may influence the hydraulic
are mostly classified as poor bedrock aquifers, as are
conditions of, and the proportion of recharge reaching,
all the Namurian strata.
the underlying bedrock aquifers.
Much of the potential recharge to these strata is
Maps of the subsoil permeability and groundwater
rejected and throughput is low. Groundwater flow is
vulnerability within the SERBD are presented in Figs
generally restricted to the upper weathered zone, to
5.2 and 5.3, respectively. The majority of the uplands
more permeable beds of limited extent or to fault
area, above 200 m elevation, has bedrock overlain by
zones. There are shallow, short localised flow
less than 3 m of subsoil (denoted in grey in Fig. 5.2).
systems, often with very little continuity between them.
Groundwater in these areas is classified as extremely
The slope of the water table generally reflects the
vulnerable. This situation covers 36% of the study
surface topography (Daly, 1994).
area.
29
Review of groundwater-level data in the SERBD
Figure 5.2. Subsoil permeability of the South Eastern River Basin District.
30
K. Tedd et al. (2008-FS-WQ-16-S4)
Figure 5.3. Groundwater vulnerability of the South Eastern River Basin District.
31
Review of groundwater-level data in the SERBD
The gravels and alluvium associated with the main
Misstear and Brown (2008) developed a quantified link
river channels are classified as being of high
between recharge and groundwater vulnerability using
permeability, as well as the Curragh Sand/Gravel
a number of case studies. From previous studies, it
Aquifer and the Screen Hills Moraine. Seven per cent
was recognised that the most important linkage to
of the bedrock of the study area is overlain by high
study was that between subsoil permeability and
permeability subsoils.
recharge. Three of the four case studies lie within the
SERBD. The recharge coefficients calculated from the
case studies were:
The shale and sandstone tills associated with the
Slieveardagh Hills, the Castlecomer Plateau and the
Lower
Palaeozoic
strata
are
classed
as
•
low
81–85% for the high permeability subsoils of the
permeability deposits, as well as the peat deposits in
Curragh (Mid-Kildare) Gravel Aquifer (Misstear et
the north and north-west. Even a thin layer of peat,
al., 2009);
especially non-cutover lowland peat, can have a
•
significant influence in reducing potential recharge
41–54% for the moderate permeability subsoils
within the Callan to Bennettsbridge Lowlands (or
(Misstear et al., 2009). Twenty-three per cent of the
36–60% for the full sub-catchment, which includes
study area is overlain by low permeability subsoils.
high and low permeability subsoils as well as
moderate permeability subsoils); and
The remaining subsoils (34%) are classified as
moderate or moderate/low permeability (where the
•
permeability mapping is not complete).
5.4
55–65% for the moderate permeability subsoils of
the Galmoy Mine area.
Recharge
5.5
The influence of subsoils on groundwater recharge in
Conceptual Model
A schematic cross-section conceptual model which
an Irish setting was investigated by Fitzsimons and
extends from the Nuenna Catchment, through the
Misstear (2006) using a simple numerical model. The
Slieveardagh
results of this work were used in the preparation of a
Hills
and
into
the
Callan
to
Bennettsbridge Lowlands is shown in Fig. 5.5. The
set of recommended values for recharge coefficients,
cross section extends from the Namurian Sandstones
for a variety of hydrogeological situations in Ireland,
and Shales, through the Dinantian Karstified and
and were contained in a report produced by the
Dolomitised
Working Group on Groundwater (2005).
Devonian–Dinantian Kiltorcan Sandstone Aquifer, and
Limestone
Aquifers
and
into
the
includes the Kilmanagh River Gravel Aquifer (see
The Working Group on Groundwater report (2005)
Fig.4.1 for location of cross section). The cross section
provides guidance on assessing the impacts of
‘dog-legs’ to include the Woodsgift, Clomantagh
groundwater abstractions during the initial phase of
Lower, Oldtown, Rathduff and Knocktopher Manor
characterisation
and
MPs. The cross section is based on geological maps
groundwater-dependent terrestrial ecosystems for the
and cross sections presented in Archer et al. (1996)
EU WFD. The report recommends that, for initial
and Daly (1994) and on the geological logs for the
characterisation, recharge should be estimated by
above-mentioned MPs.
of
groundwater
bodies
multiplying the estimated effective rainfall value by the
recharge coefficient. In areas underlain by poor
Included on the cross section are three typical
aquifers, a recharge ‘cap’ should be applied. Further
hydrographs of aquifers located from the recharge to
details are given in Box 5.2.
the discharge zone. It should be noted that aquifer
units are likely to be limited in extent due to faulting and
A recharge map of Ireland was developed using the
the nature of fracture flow; therefore, the hydrographs
resulting recharge values. The recharge map for the
in Fig. 5.5 do not represent a regional groundwater
SERBD is presented in Fig. 5.4.
flow.
32
K. Tedd et al. (2008-FS-WQ-16-S4)
Figure 5.4. Recharge map for the South Eastern River Basin District.
33
King’s River
Figure 5.5. Schematic conceptual model of a cross section through the South Eastern River Basin District.
Review of groundwater-level data in the SERBD
34
BT
K. Tedd et al. (2008-FS-WQ-16-S4)
Box 5.2. Recharge Coefficients
The proportion of effective rainfall (ER) that becomes recharge is referred to as the recharge coefficient. This
depends largely on the permeability and thickness of the subsoils and unsaturated bedrock overlying the
groundwater, as well as on the presence of ‘wet’ and ‘dry’ soils or peat.
Recharge estimation methodology (recommended for the initial characterisation):
•
Estimate effective rainfall.
•
Multiply effective rainfall by the appropriate recharge coefficient to give the potential recharge.
•
In areas underlain by poor aquifers, apply a maximum recharge ‘cap’ (see below). This takes account of the
limited capacity of such aquifers to accept recharge.
•
Where point recharge is present, use information on the local situation to estimate its significance and the likely
catchment area of the point.
•
If possible, corroborate results with an assessment of base flow from local rivers.
Where further characterisation is required, a more comprehensive assessment of recharge will be necessary, which
will require more detailed analysis of ER and base flow.
Recharge caps
•
In areas underlain by poor aquifers (Pl and Pu), the maximum recharge should be taken as 100 mm/year.
•
In areas underlain by locally important aquifers that are generally unproductive except for local zones (Ll), the
maximum recharge should be in the range of 150–200 mm/year, depending upon local knowledge.
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